Regulation of thylakoid protein phosphorylation in intact chloroplasts by the activity of kinases and phosphatases

Dark recovery of the Chl a fluorescence transient (OJIP) after light adaptation: The qT-component of non-photochemical quenching is related to an activated photosystem I acceptor side

The dark recovery kinetics of the Chl a fluorescence transient (OJIP) after 15 min light adaptation were studied and interpreted with the help of simultaneously measured 820 nm transmission. The kinetics of the changes in the shape of the OJIP transient were related to the kinetics of the qE and qT components of non-photochemical quenching. The dark-relaxation of the qE coincided with a general increase of the fluorescence yield. Light adaptation caused the disappearance of the IP-phase (20-200 ms) of the OJIP-transient. The qT correlated with the recovery of the IP-phase and with a recovery of the re-reduction of P700(+) and oxidized plastocyanin in the 20-200 ms time-range as derived from 820 nm transmission measurements. On the basis of these observations, the qT is interpreted to represent the inactivation kinetics of ferredoxin-NADP(+)-reductase (FNR). The activation state of FNR affects the fluorescence yield via its effect on the electron flow. The qT therefore represents a form of photochemical quenching. Increasing the light intensity of the probe pulse from 1800 to 15000 mumol photons m(-2) s(-1) did not qualitatively change the results. The presented observations imply that in light-adapted leaves, it is not possible to 'close' all reaction centers with a strong light pulse. This supports the hypothesis that in addition to Q(A) a second modulator of the fluorescence yield located on the acceptor side of photosystem II (e.g., the occupancy of the Q(B)-site) is needed to explain these results. Besides, some of our results indicate that in pea leaves state 2 to 1 transitions may contribute to the qI-phase.

Transient decrease of light-harvesting complex II phosphorylation level by hypoosmotic shock in dark-adapted Dunaliella salina

This study investigated the regulation of major light harvesting chlorophyll a/b protein (LHCII) phosphorylation by hypoosmotic shock in dark-adapted Dunaliella salina cells. When the external NaCl concentration decreased in darkness, D. salina LHCII phosphorylation levels transiently dropped within 20 min and then restored gradually to basal levels. The transient decrease in LHCII phosphorylation levels was insensitive to NaF, a phosphatase inhibitor. Inhibition of intracellular ATP production by addition of an uncoupler or an ATP synthase inhibitor increased LHCII phosphorylation levels in D. salina cells exposed to hypoosmotic shock. Taken together, these results indicate that hypoosmotic shock inhibits the LHCII phosphorylation process. The related mechanism and physiological significance are discussed.

Functional flexibility and acclimation of the thylakoid membrane

Light is an elusive substrate for the function of photosynthetic light reactions of photosynthesis in the thylakoid membrane. Therefore structural and functional dynamics, which occur in the timescale from seconds to several days, are required both at low and high light conditions. The best characterized short-time regulation mechanism at low light is a rapid state transition, resulting in higher absorption cross section of PSI at the expense of PSII. If the low light conditions continue, activation of the lhcb-genes and synthesis of the light-harvesting proteins will occur to optimize the functions of PSII and PSI. At high light, the transition to state 2 is completely inhibited, but the feedback de-excitation of absorbed energy as heat, known as the energy-dependent quenching (q(E)), is rapidly set up. It requires, at least, the DeltapH-dependent activation of violaxanthin de-epoxidase and involvement of the PsbS protein. Another crucial mechanism for protection against the high light stress is the PSII repair cycle. Furthermore, the water-water cycle, cyclic electron transfer around PSI and chlororespiration are important means induced under high irradiation, functioning mainly to avoid an excess production of reactive oxygen species.

Redox regulation of thylakoid protein phosphorylation

The photosystem II of chloroplast thylakoid membranes contains several proteins phosphorylated by redox-activated protein kinases. The mechanism of the reversible activation of the light-harvesting antenna complex II (LHCII) kinase(s) is one of the best understood and related to the regulation of energy transfer to photosystem II or I, thereby optimizing their relative excitation (state transition). The deactivated LHCII protein kinase(s) is associated with cytochrome b(6)f and dissociates from the complex upon activation. Activation of the LHCII protein kinase occurs via dynamic conformational changes in the cytochrome b(6)f complex taking place during plastoquinol oxidation. Deactivation of the kinase involves its reassociation with an oxidized cytochrome complex. A fine-tuning redox-dependent regulatory loop inhibits the activation of the kinase via reduction of protein disulfide groups, possibly involving the thioredoxin complex. Phosphorylation of LHCII is further modulated by light-induced conformational changes of the LHCII substrate. The reversible phosphorylation of LHCII and other thylakoid phosphoproteins, catalyzed by respective kinases and phosphatases, is under strict regulation in response to environmental changes.

Balance of power: a view of the mechanism of photosynthetic state transitions

Photosynthesis in plants involves photosystem I and photosystem II, both of which use light energy to drive redox processes. Plants can balance the distribution of absorbed light energy between the two photosystems. When photosystem II is favoured, a mobile pool of light harvesting complex II moves from photosystem II to photosystem I. This short-term and reversible redistribution is known as a state transition. It is associated with changes in the phosphorylation of light harvesting complex II but the regulation is complex. Redistribution of energy during state transitions depends on an altered binding equilibrium between the light harvesting complex II-photosystem II and light harvesting complex II-photosystem I complexes.

Primary structure characterization of the photosystem II D1 and D2 subunits

Mass spectrometry techniques have been applied in a protein mapping strategy to elucidate the majority of the primary structures of the D1 and D2 proteins present in the photosystem II reaction center. Evidence verifying the post-translational processing of the initiating methionine residue and acetylation of the free amino group, similar to those reported for other higher plant species, are presented for the two subunits from pea plants (Pisum sativum L.). Further covalent modifications observed on the D1 protein include the COOH-terminal processing with a loss of nine amino acids and phosphorylation of Thr2. In addition, the studies reported in this paper provide the first definitive characterization of oxidations on specific amino acids of the D1 and D2 proteins. We believe that these oxidations, and to a much lesser extent the phosphorylations, are major contributors to the heterogeneity observed during the electrospray analysis of the intact subunits reported in the accompanying paper (Sharma, J., Panico, M., Barber, J., and Morris, H. R. (1997) J. Biol. Chem. 272, 33153-33157). Significantly, all of the regions that have been identified as those particularly susceptible to oxidation are anticipated (from current models) to be in close proximity to the redox active components of the photosystem II complex.

Phosphorylation of light-harvesting complex II and photosystem II core proteins shows different irradiance-dependent regulation in vivo. Application of phosphothreonine antibodies to analysis of thylakoid phosphoproteins

An immunological approach using a polyclonal phosphothreonine antibody is introduced for the analysis of thylakoid protein phosphorylation in vivo. Virtually the same photosystem II (PSII) core phosphoproteins (D1, D2, CP43, and the psbH gene product) and the light-harvesting chlorophyll a/b complex II (LHCII) phosphopolypeptides (LHCB1 and LHCB2), as earlier identified by radiolabeling experiments, were recognized in both pumpkin and spinach leaves. Notably, the PSII core proteins and LHCII polypeptides were found to have a different phosphorylation pattern in vivo with respect to increasing irradiance. Phosphorylation of the PSII core proteins in leaf discs attained the saturation level at the growth light intensity, and this level was also maintained at high irradiances. Maximal phosphorylation of LHCII polypeptides only occurred at low light intensities, far below the growth irradiance, and then drastically decreased at higher irradiances. These observations are at variance with traditional studies in vitro, where LHCII shows a light-dependent increase in phosphorylation, which is maintained even at high irradiances. Only a slow restoration of the phosphorylation capacity for LHCII polypeptides at the low light conditions occurred in vivo after the high light-induced inactivation. Furthermore, if thylakoid membranes were isolated from the high light-inactivated leaves, no restoration of LHCII phosphorylation took place in vitro. However, both the high light-induced inactivation and low light-induced restoration of LHCII phosphorylation seen in vivo could be mimicked in isolated thylakoid membranes by incubating with reduced and oxidized dithiothreitol, respectively. We propose that stromal components are involved in the regulation of LHCII phosphorylation in vivo, and inhibition of LHCII phosphorylation under increasing irradiance results from reduction of the thiol groups in the LHCII kinase.

Stabilization of photosystem two dimers by phosphorylation: implication for the regulation of the turnover of D1 protein

A general feature of many membrane protein complexes is that they have oligomeric organisation in vivo. Photosystem II (PSII) is one such example and the possible functional significance of this is explored in this work. Monomeric and dimeric forms of the core complex of PSII have been isolated from non-phosphorylated and phosphorylated thylakoid membranes prepared from spinach. These complexes had the same complement of proteins including, D1 (PsbA), D2 (PsbD), alpha-(PsbE) and beta-(PsbF) subunits of cytochrome b559, CP47 (PsbB), CP43 (PsbC), 33 kDa (PsbO) extrinsic protein and some other smaller subunits, such as PsbH, but did not contain Cab proteins. D1, D2, CP43 and PsbH were the phosphorylated components. Whether phosphorylated or not, the dimeric form of the PSII complex was more stable than the monomeric form. However, when treated with photoinhibitory light the isolated dimers converted to monomers in their non-phosphorylated state but not when phosphorylated. Phosphorylation, however, did not prevent photoinhibition as judged by the loss of oxygen evolving activity. A model is suggested for the role of PSII phosphorylation in controlling the conversion of dimeric PSII to its monomeric form and in this way regulate the rate of degradation of D1 protein during the photoinhibitory repair cycle.

Plastoquinol at the quinol oxidation site of reduced cytochrome bf mediates signal transduction between light and protein phosphorylation: thylakoid protein kinase deactivation by a single-turnover flash

Redox-controlled phosphorylation of thylakoid membrane proteins represents a unique system for the regulation of light energy utilization in photosynthesis. The molecular mechanisms for this process remain unknown, but current views suggest that the plastoquinone pool directly controls the activation of the kinase. On the basis of enzyme activation by a pH shift in the darkness combined with flash photolysis, EPR, and optical spectroscopy we propose that activation occurs when plastoquinol occupies the quinol-oxidation (Qo) site of the cytochrome bf complex, having its high-potential path components in a reduced state. A linear correlation between kinase activation and accessibility of the Qo site to plastoquinol was established by quantification of the shift in the g(y) EPR signal of the Rieske Fe-S center resulting from displacement of the Qo-site plastoquinol by a quinone analog. Activity persists as long as one plastoquinol per cytochrome bf is still available. Withdrawal of one electron from this plastoquinol after a single-turnover flash exciting photosystem I leads to deactivation of the kinase parallel with a decrease in the g(z) EPR signal of the reduced Rieske Fe-S center. Cytochrome f, plastocyanin, and P(700) are rereduced after the flash, indicating that the plastoquinol at the Qo site is limiting in maintaining the kinase activity. These results give direct evidence for a functional cytochrome bf-kinase interaction, analogous to a signal transduction system where the cytochrome bf is the receptor and the ligand is the plastoquinol at the Qo site.

Phosphorylation of PS II polypeptides inhibits D1 protein-degradation and increases PS II stability

To study the significance of Photosystem (PS) II phosphorylation for the turnover of the D1 protein, phosphorylation was compared with the synthesis and content of the D1 protein in intact chloroplasts. As shown by radioactive labelling with [(32)Pi] phosphorylation of PS II polypeptides was saturated at light intensities of 125 mol m(-2) s(-1). Under steady state conditions, in intact chloroplasts D1 protein, once it was phosphorylated, was neither dephosphorylated nor degraded in the light. D1 protein-synthesis was measured as incorporation of [(14)C] leucine. As shown by non-denaturing gel-electrophoresis followed by SDS-PAGE newly synthesised D1 protein was assembled to intact PS II-centres and no free D1 protein could be detected. D1 protein-synthesis was saturated at light intensities of 500 mol m(-2) s(-1). The content of D1 protein stayed stable even after illumination with 5000 μmol m(-2) s(-1) showing that D1 protein-degradation was saturated at the same light intensities. The difference in the light saturation points of phosphorylation and of D1 protein-turnover indicates a complex regulation of D1 protein-turnover by phosphorylation. Separation of the phosphorylated and dephosphorylated D1 protein by LiDS-gelelectrophoresis combined with radioactive pulse-labelling with [(14)C] leucine and [(32)Pi] revealed that D1 protein, synthesised under steady state conditions in the light, did not become phosphorylated but instead was rapidly degraded whereas the phosphorylated form of the D1 protein was not a good substrate for degradation. According to these observations phosphorylation of the D1 protein creates a pool of PS II centres which is not involved in D1 to these observations phosphorylation of the D1 protein creates a pool of PS II centres which is not involved in D1 protein-turnover. Fractionation of thylakoid membranes confirms that the phosphorylated, non-turning over pool of PS II-centres was located in the central regions of the grana, whereas PS II-centres involved in D1 protein-turnover were found exclusively in the stroma-lamellae and in the grana-margins.

Differential D1 dephosphorylation in functional and photodamaged photosystem II centers. Dephosphorylation is a prerequisite for degradation of damaged D1

Light dependence and kinetics of reversible phosphorylation of the D1 reaction center protein of Photosystem II was studied in pumpkin leaves. At growth light, maximal phosphorylation of D1 was observed after illumination of 1 h, with higher phosphorylation rates at stronger irradiances. 70-85% of D1 became phosphorylated, corresponding to the proportion of the protein in appressed thylakoid membranes. Comparison of the kinetics of D1 phosphorylation and photoinactivation of Photosystem II revealed that D1 phosphorylation became saturated before any significant photoinhibition of Photosystem II could be detected. Dephosphorylation of D1 in both dim light and darkness was determined in leaves preilluminated with high light for various periods. Similar rates of D1 dephosphorylation were observed after short preillumination conditions that induce no significant loss of functional Photosystem II centers. In contrast, photodamage to Photosystem II centers significantly decreased the dephosphorylation rate of D1 in darkness, and no dephosphorylation occurred in leaves containing mainly damaged Photosystem II centers. Darkness also blocked the degradation of damaged D1 after photoinhibitory preillumination. Degradation of damaged D1 could be prevented even in dim light by sodium fluoride, an inhibitor of protein phosphatases, indicating that dephosphorylation is a prerequisite for D1 proteolysis. We conclude that in higher plants (i) high light induced photodamage to Photosystem II occurs in the centers containing phosphorylated D1. (ii) Dephosphorylation of phosphorylated and photodamaged D1 is associated with the repair cycle of inactivated Photosystem II and is a light-dependent reaction in vivo. (iii) Dephosphorylation of D1 in functional Photosystem II centers, however, occurs rapidly and independent of light. We suggest that two reversible phosphorylation cycles with spatially segregated protein phosphatases are involved in dephosphorylation of functional and damaged phosphorylated D1, respectively.

Phosphatase activities in spinach thylakoid membranes-effectors, regulation and location

The dephosphorylation of seven phosphoproteins associated with Photosystem II or its chlorophyll a/b antenna in spinach thylakoids, was characterised. The rates were found to fall into two distinct groups. One, rapidly dephosphorylated, consisted of the two subunits (25 and 27 kD) of the major light harvesting complex of Photosystem II (LHC II) and a 12 kD polypeptide of unknown identity. A marked correlation between the dephosphorylation of these three phosphoproteins, strongly suggested that they were all dephosphorylated by the same enzyme. Within this group, the 25 kD subunit was consistently dephosphorylated most rapidly, probably reflecting its exclusive location in the peripheral pool of LHC II. The other group, only slowly dephosphorylated, included several PS II proteins such as the D1 and D2 reaction centre proteins, the chlorophyll-a binding protein CP43 and the 9 kD PS II-H phosphoprotein. No dephosphorylation was observed in either of the two groups in the absence of Mg(2+)-ions. Dephosphorylation of the two LHC II subunits took place in both grana and stroma-exposed regions of the thylakoid membrane. However, deposphorylation in the latter region was significantly more rapid, indicating a preferential dephosphorylation of the peripheral (or 'mobile') LHC II. Dephosphorylation of LHC II was found to be markedly affected by the redox state of thiol-groups, which may suggest a possible regulation of LHC II dephosphorylation involving the ferredoxin-thioredoxin system.

Activation/deactivation cycle of redox-controlled thylakoid protein phosphorylation. Role of plastoquinol bound to the reduced cytochrome bf complex

Signal transduction via light-dependent redox control of reversible thylakoid protein phosphorylation has evolved in plants as a unique mechanism for controlling events related to light energy utilization. Here we report for the first time that protein phosphorylation can be activated without light or the addition of reducing agents by a transient exposure of isolated thylakoid membranes to low pH in darkness. The activation of the kinase after incubation of dark-adapted thylakoids at pH 4.3 coincides with an increase in the plastoquinol: plastoquinone ratio up to 0.25. However, rapid plastoquinol reoxidation ( < 1 min) at pH 7.4 contrasts with the slow kinase deactivation (t 1/2 = 4 min), which indicates that the redox control is not directly dependent on the plastoquinone pool. Use of inhibitors and a cytochrome bf-deficient mutant of Lemna demonstrate the involvement of the cytochrome bf complex in the low-pH induced protein phosphorylation. EPR spectroscopy shows that subsequent to the transient low pH treatment and transfer of the thylakoids to pH 7.4, the Rieske Fe-S center, and plastocyanin become reduced and are not reoxidized while the kinase is slowly deactivated. However, the deactivation correlates with a decrease of the EPR gz signal of the reduced Rieske Fe-S center, which is also affected by quinone analogues that inhibit the kinase. Our data point to an activation mechanism of thylakoid protein phosphorylation that involves the binding of plastoquinol to the cytochrome bf complex in the vicinity of the reduced Rieske Fe-S center.

Photoinactivation of photosystem II induces changes in the photochemical reaction center II abolishing the regulatory role of the QB site in the D1 protein degradation

The effect of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (diuron) binding at the secondary quinone (QB) binding site of reaction center II (RCII), on the high-light-induced degradation of the RCII proteins D1 and D2, and the core proteins CP43 and CP47 was investigated in vivo in Chlamydomonas reinhardtii. The degradation of the RCII-D2 and the CP43 proteins shows a short lag relative to that of the RCII-D1 protein. Diuron retards but does not prevent the degradation of RCII-D1, D2 and CP43 proteins. The degradation of the CP47 protein is not retarded by diuron. The RCII-D1 protein present in cells photoinactivated in the presence of diuron is subsequently degraded in cells transferred to low light or to darkness. The protein can be replaced (turnover) at least partially under both conditions. The RCII-D1 protein is not degraded during photoinactivation of a cytochrome-bf-defective mutant. Degradation occurs however when the cells are returned to low light permitting slow reoxidation of plastoquinol [Zer, H., Prasil, O. & Ohad, I. (1994) J. Biol. Chem. 269, 17,670-17,676]. Addition of diuron does not prevent the degradation of the protein at this stage. Tryptic digestion of the RCII-D1 protein is partially inhibited by diuron in isolated thylakoids [Trebst, A., Depka, B., Kraft, B. & Johanningmeier, U. (1988) Photosynth. Res. 18, 163-177] but not in thylakoids obtained from photoinactivated cells. We conclude that photoinactivation induces a series of sequential changes in RCII exposing the cleavage site of the RCII-D1 protein to degradation and abolishing the regulatory role of the QB site occupancy by plastoquinone or analog ligands on the cleavage process. The degradation of the RCII-D2 and CP43 proteins may be a secondary process following modification and/or loss of the RCII-D1 protein.